Heat related defects are an ongoing problem in the shaping of heated materials and in casting processes in particular. Increased complexity of cast parts, demands for higher productivity, and demands for lower scrap rates all tend to place higher thermal loads on die cooling systems. Typical cooling systems usually consist of a number of channels drilled or cut into the tool through which a liquid coolant is intended to flow. This is a single-phase system, where heat is transferred from the die surface (which is in contact with the material being shaped), through the material of the die, and to the surface of the cooling channels, where the liquid cooling fluid absorbs heat and carries it away from the die.
Heat is removed from the cooling channel walls by the cooling fluid by forced convection in an attempt to maintain an isothermal condition at the shaping surface. However, due to the high heat flux at the interface between the heated material and the die, high heat diffusion resistance of the die material (e.g., tool steel), and the convective resistance of the cooling fluid, more heat is sometimes transferred into the die material than is removed by the cooling fluid during shaping and cooling portions of the process. This leads to an increase in the overall temperature of the die, which can cause of a number of heat related problems, such as die soldering, shrink porosity, and/or surface cracks in the shaped part. With conventional cooling systems, excess heat is sometimes removed using die spray after the solidification portion of the casting process. This can lengthen cycle times, increase lubricant use, increase liquid reclaim cost, and reduce die tool life via failure of the tool steel.